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A major achievement of 20th century cell biology was the identification of the membrane cytoskeleton in mammalian red blood cells (RBCs) (1). This cytoskeletal network is comprised of long, flexible (α1β1)2 spectrin tetramers that are linked together by short, actin filament-based, junctional complexes to form a 2D, quasi-hexagonal lattice (2, 3). This lattice is then tethered to the RBC’s plasma membrane by numerous interactions, most notably the ankyrin B-dependent interaction of spectrin with band 3, an abundant transmembrane protein in RBCs. The RBC’s characteristic biconcave shape, as well as its physical properties—strong enough to withstand high shear forces, yet flexible enough to pass through capillaries half its diameter—are made possible by this membrane cytoskeleton. Two remarkable facts about the actin filament-based junctional complexes in this network are that they contain ∼96% of the RBC’s total actin, and that the single actin filament present in each complex is always ∼37 nm in length. Precise length control is created by the presence of capping proteins at the barbed and pointed ends of the junctional actin filament (αβ-adducin and tropmodulin, respectively), and two “short” tropomyosin isoforms (TM5b and TM5NM1) that extend along most of the junctional filament’s length. Importantly, the uniform length of junctional actin filaments permits just six spectrin attachments per filament, thereby promoting the network’s quasi-hexagonal symmetry. While junctional actin filaments are generally considered to be quite static, especially relative to actin filaments in the cortex of typical cells, they do undergo subunit exchange.

When considering actin function in many cellular contexts, nonmuscle myosin 2 (NM2) naturally comes to mind, as these motors are the major actin-based contractile machines in most cell types (4, 5). Consistently, NM2s power a wide range of fundamental cellular processes, including cell migration, cytokinesis, tissue morphogenesis, and epithelial barrier …

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